The present invention relates to an image forming apparatus for exposing light emitted from an organic-EL (electro luminescence) photo emitter array onto a photosensitive member by an image forming optical system.
In an image forming apparatus such as a copying machine, a printer or a facsimile using an electrophotographic technique, conventionally, there have been known an image forming apparatus for scanning a laser beam irradiated from a laser beam source by using a rotary polygon mirror to form an electrostatic latent image on a photosensitive member and an image forming apparatus for forming an electrostatic latent image on a photosensitive member by using an LED (light emitting diode) array having a plurality of LEDs arranged linearly. Moreover, there has also been known an image forming apparatus having an organic-EL photo emitter arranged linearly. Exemplified configurations are disclosed in Japanese Patent Publication Nos. 2-273258A, 11-198433A, 2000-103114A and 2004-50816A.
In case of an exposing method using a laser beam, however, there is a problem in that an optical component such as a polygon mirror or a lens is required, it is hard to reduce the size of an apparatus and it is also hard to increase an operation speed largely. In a color image forming apparatus having a tandem arrangement, furthermore, it is hard to enhance precision in the scanning positions of four scanning beams. For this reason, there is a problem in that a color deviation is generated, resulting in a deterioration in image quality.
In case of an exposing method using an LED array in which a large number of minute LEDs are arranged linearly, the LED can generally obtain a high luminance. However, the LED is basically manufactured by using a semiconductor process. For this reason, a substrate is expensive and a substrate length cannot be increased in respect of a manufacturing yield. Therefore, it is necessary to arrange a large number of chips in a line to form the LED array. In that case, there is a problem in that a step between the chips and the error of a space cause a variation in the luminance and a density unevenness remarkably appears on an image.
An organic-EL photo emitter using an organic polymer substance for a light emitting layer is easier to manufacture and has a smaller amount of heat generation as compared with other photo emitters. Therefore, a radiation fin for cooling can be omitted and the thickness and weight of an exposer can be reduced. However, the light emitting wavelength of the organic-EL photo emitter has a large half-value width of approximately 100 nm. Therefore, an effective wavelength distribution (a light emitting wavelength distribution seen from a photosensitive member) also has a great half-value width of approximately 100 nm.
FIG. 13 shows the state that an image formation is performed on a photosensitive member 2 by irradiating light emitted from a light emitting section 8 of the organic-EL photo emitter 6 through a rod lens 3. The organic-EL photo emitter has a great light emitting wavelength distribution. For this reason, a beam having a long wavelength shown in a dashed line forms an image in a great spot diameter on the photosensitive member 2 by the chromatic aberration of the rod lens 3.
FIG. 14 is a chart showing the comparison of a light emitting intensity and a spot diameter between light having a single wavelength and light having a large wavelength distribution as in the organic-EL photo emitter 6. The spot diameter (a dashed line) of the organic-EL photo emitter 6 having a large wavelength distribution is greater than the spot diameter (a solid line) of the light having a single wavelength. As a result, there is a problem in that a clear image is formed with difficulty. When the spot diameter is increased, the contour of an image is not clear so that image quality is influenced.
Japanese Patent Publication Nos. 2000-103114A and 2004-50816A disclose a line head having a plurality of organic-EL photo emitters arrayed in the primary scanning direction of a photosensitive member (note: this term is defined when a scanning optics is used to irradiate the photosensitive member), and a plurality of arrays are arranged in the secondary scanning direction of the photosensitive member (note: also defined as a moving direction of the photosensitive member) for carrying out a multiple exposure over the same pixel on the photosensitive member.
In a case where the organic-EL photo emitters in each of the arrays are controlled with the PWM (pulse width modulation) technique to perform a gradation reproduction, the spots of the organic-EL photo emitters in the arrays are superposed on the same pixel on the photosensitive member and are thus exposed to light. Therefore, precision in the shape of the spot of the photosensitive member greatly influences image quality. In a case where a pitch between pixel lines in the secondary scanning direction is small, particularly, a blank portion between the adjacent lines is lost so that the quality of an image is deteriorated when the spot diameter in the secondary scanning direction is increased.